1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a backlight unit that achieves a half-power angle of 40° without having a polarizing sheet.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls the transmittance of light beams emitted from a backlight unit through a liquid crystal display panel. The liquid crystal display panel includes a plurality of liquid crystal cells arranged in a matrix and a plurality of control switches for supplying video signals to the corresponding liquid crystals cell in order to display a desired picture on a screen.
FIG. 1 is a side view schematically illustrating a backlight unit according to related art. Referring to FIG. 1, the related art backlight unit includes a light guide panel 4 guiding light beams from a light emission part; a reflecting plate 2 under the light guide panel 4 for reflecting, towards the upper surface, light beams that progress to the lower and side surfaces of the light guide panel 4; and a diffusion sheet 6 for diffusing the light beams that have passed through the light guide panel 4. The backlight unit further includes a prism sheet 8 for controlling the progress directions of the light beams that have passed through the diffusion sheet 6; and a polarizing sheet 10 for selectively transmitting the light beams that have passed through the prism sheet 8.
The light emission part includes a lamp 14 for generating light beams and a lamp housing 16 for mounting the lamp 14 and for reflecting the light beams from the lamp 14 towards the light guide panel 4. Substantially all of the light beams generated in the light emission part are directed towards the upper surface of the light guide panel 4 by utilizing the total reflection condition of the light beams that reach a printed pattern formed at the lower surface of the light guide panel 4. The light beams that have progressed to the lower surface and the side surface of the light guide panel 4 are reflected by the reflecting plate 2 to progress in the direction of the upper surface of the light guide panel 4. The light beams that have passed through the light guide panel 4 are diffused by a diffusion sheet 6.
The light beams incident to the liquid crystal display panel 12 will have a higher light efficiency when the incident direction is more perpendicular to the surface of the panel 12. For this purpose, the prism sheet 8 and the polarizing sheet 10 are deposited on the diffusion sheet 6 to make the light beams perpendicular to the surface of the liquid crystal display panel 12. As shown in FIG. 1, the light beams that have passed through the prism sheet 8 are incident on the liquid crystal display panel 12 via the polarizing sheet 10.
Because the polarizing sheet 10 is coated with a linear reflex polarizing material such as a double brightness enhancement film (DBEF) material, it can operate as a mirror in one condition, and operate as an image screen in another condition. The linear reflex polarizing material transmits substantially all of the linear polarized light components that are polarized in one direction (the transmitting direction) and reflects substantially all of the linear polarized light components that are polarized in a direction perpendicular the transmitting direction.
FIG. 2 is a detailed drawing of the related art prism sheet of FIG. 1. Referring to FIG. 2, the related art prism sheet 8 includes a condensing film 8b of polyester PET, and a lenticular layer 8a that extends in a specific direction on the upper surface of the condensing film 8b. The condensing film 8b is formed with its thickness 155 μm and the lenticular layer 8a is composed of a plurality of linear prisms. Each of the linear prisms has two inclined surfaces about an apex. The distance P1 between the two adjacent two apexes (i.e., the pitch) is 50 μm, the apex angle θ between the two inclines is 90°.
In the related art backlight unit, however, due to the use of an expensive polarizing sheet, manufacturing costs and the number of the required processes are undesirably large.